James Mattis worked his way up through the military ranks, served as a rifle and weapons platoon commander and later ran a recruiting station in Portland. He commanded assault battalions in the Gulf War (1990-91), and more recently led ground forces in the wars against both Afghanistan and Iraq. Mattis led US troops in the Battle of Fallujah in Iraq (2004), a confrontation regarded as particularly brutal even by standards of war, in which Mattis personally altered the rules of engagement to reverse the priority from "capture or kill" to "kill or capture."

He is widely respected and admired in the military, and seen as brusque but brilliant. He is known for frequent visits to the front lines during battle, and he has described his job as simply "killing the enemy." Mattis drew political fire and a gentle rebuke from higher-ups for a 2005 speech in which he offered his perspective bluntly but colorfully: "You go into Afghanistan, you got guys who slap women around for five years because they didn't wear a veil. You know, guys like that ain't got no manhood left anyway. So it's a hell of a lot of fun to shoot them." He ships a large library of books to wherever he is assigned, frequently quotes Shakespeare, Sun Tzu, and Clausewitz, and has offered admiring analysis of tactics used by Native American warriors in the 19th century.

Nominated 8 July 2010 to become Commander of the US Central Command (CENTCOM).